In power electronics, arrangements like static converter devices the current comutates between valve arms, and each valve arm is arranged in the device so that current flow should only be possible in the forward direction. In the opposite direction (reverse direction), the valve arm should block current. In a symmetrically blocking GTO, the blocking is inherent. Symmetrical blocking means blocking current independent of the anode-to-cathode voltage, when controlled by a blocking signal. An asymmetrically blocking GTO is capable of blocking a current when there is a positive anode-to-cathode voltage, but possesses no blocking capability when there is reverse voltage polarity. In a transistor or asymmetrically blocking GTO, blocking in the reverse direction is provided by a serially connected diode. To commutate the current, the valve device of the valve arm delivering "current is blocked (disconnected"), and the valve device of the valve arm which assumes delivery of the current (the "current-assuming valve arm") is actuated.
If bridge connections with disconnectable valve arms that are capable of being blocked in the reverse direction are operated in the range of natural commutation, then it is sufficient to actuate the current-assuming valve arm, whereby the current-delivering valve arm receives negative anode-to-cathode voltage and is extinguished. Initially a reverse current forms, which subsequently falls away sharply. However, an overvoltage results, which is dependent on the parasitic inductance of the valve damping circuit and can have values that are so high that the valve becomes endangered.
To avoid this danger to the valves, an especially low-resistance and low-inductance type of valve damping circuit, known in standard thyristor circuits, can be applied. However, this circuit stresses the valve when it is actuated, as a result of a steep current surge.
In FIG. 1, half of a known bridge connection is depicted. A virtually constant, load-independent D/C current is impressed on this bridge connection half via the inductor DR. It contains, for example, three bridge arms, each with a valve device. These valve devices each have transistors T1, T3, T5 with series diodes D1, D3, D5. Through amplification and pulse shaping, control circuits AS1, AS3, AS5 form control voltages for the gate electrodes of the valves from the appropriate control signals s1, s3, s5. An RCD-network is connected in parallel with the transistor-diode series connection. The RCD-network has considerable parasitic inductance, as indicated graphically in FIG. 1. The control circuit and RCD-protective circuit are standard in the industry and are known, for example, from Appendix 2 to the "Thyristor Applications Notes" AN-315 (U.S.A., 1982) of the firm "International Rectifier."
As long as the valve T1 carries a current, with valves T3 and T5 blocked, the voltage of the direct current connection P is coupled to the three-phase a.c. current connection R of the bridge connection up to the output voltage of the valve arm. If now, for the polarity of the three-phase a.c. current connections as drawn in FIG. 1, appropriate control signals s1 and s3 are used to simultaneously actuate the valve arm T3 and block the valve arm T1, then valve T3 receives (or assumes) the current. However, a reverse current forms on valve T1 and a polarity reversal of the voltage occurs in the valve damping circuit RCD. If this reverse current breaks away, then the leakage inductance produces a high peak in the now negative anode-to-cathode voltage of valve T1, which thereby becomes endangered.
A method and arrangement are needed to commutate a load-independent current in a static converter while avoiding this endangerment of the valve devices in the converter.